Various methods have been conventionally known for producing a cycloolefin. As one of the methods, a method of subjecting a monocyclic aromatic hydrocarbon to partial hydrogenation performed in a liquid phase using a ruthenium catalyst is known. In producing a cycloolefin by partial hydrogenation of a monocyclic aromatic hydrocarbon, a catalyst component, a type of carrier, a metal salt as an additive to a reaction system, and the like have been investigated in order to improve selectivity of a cyclohexene or yield and the results thereof have been reported in many documents.
For example, a method (Patent Literature 1) for producing a cycloolefin is known, in which a reaction is performed by using a hydrogenation catalyst particle containing a ruthenium metal having an average crystallite diameter of 20 nm or less and adding at least one compound selected from an oxide, a hydroxide and a hydrate of Zr, Hf, Ti, Nb, Ta, Cr, Fe, Co, Al, Ga and Si besides the catalyst particle, further in the copresence of at least one type of zinc compound serving as a co-catalyst and under neutral or acidic conditions. Furthermore, for example, a catalyst for producing a cycloolefin (Patent Literature 2) is known, which is a catalyst employing zirconia as a carrier and formed of particles having an average particle size of a primary particle within the range of 3 to 50 nm and a secondary particle size within the range of 0.1 to 30 μm.
In addition, a pretreatment method (Patent Literature 3) for a ruthenium catalyst is proposed for separating an aqueous phase consisting of a ruthenium catalyst and water from a reaction product without fail. In the method, the aqueous phase containing a ruthenium catalyst is held while stirring at a temperature of 60 to 180° C. in a predetermined time in the absence of an oil-phase component such as a raw-material aromatic hydrocarbon.
Meanwhile, a method of regenerating a ruthenium catalyst reduced in activity by the interaction between hydrogen and the ruthenium catalyst is proposed (Patent Literature 4). In this method, the ruthenium catalyst is brought into contact with oxygen in a liquid phase.
Furthermore, an activity recovery method for a ruthenium catalyst is proposed (Patent Literature 5). In this method, a ruthenium catalyst used in a hydrogenation reaction of an unsaturated organic compound and reduced in activity is held at a hydrogen partial pressure, which is lower than the hydrogen partial pressure of the hydrogenation reaction conditions, and at a temperature not less than the temperature of the hydrogenation reaction minus 50° C. and not more than 250° C.
Moreover, an activity recovery method for a ruthenium catalyst is proposed (Patent Literature 6). This method includes a step of bringing a ruthenium catalyst into contact with oxygen in a liquid phase and a step of holding the catalyst at a hydrogen partial pressure lower than the hydrogen partial pressure in the hydrogenation reaction and at a temperature not less than the temperature of the hydrogenation reaction temperature minus 50° C.